1. Field of the Invention
The present invention relates to a pellicle pasting system and a pellicle pasting method, and more particularly to a pellicle pasting system and method to paste pellicle on a mask (reticle).
2. Discussion of the Related Art
In recent years, integrated circuit (IC) manufacturing has become finer and more advanced. Accordingly, the prevention of foreign objects, such as minute dust and dirt, from attaching to surfaces during manufacturing has become extremely important.
As a result, a pellicle pasted mask having a light transmitting thin film called pellicle (foreign object prevention film) is used widely today to prevent attachment of foreign objects on the mask before a circuit pattern is formed on the mask. The pellicle is then transferred onto an exposed substrate using an exposure apparatus during a photo-lithography step in the IC manufacturing process.
In this way, attachment of foreign objects on the mask is prevented beforehand by spreading the pellicle on a pellicle frame, fixing the pellicle frame and the mask by bonding, and mounting the pellicle frame to cover the mask surface. In this case, if foreign objects are attached to the pellicle, even if the mask is free of foreign objects, the foreign objects will fall on the mask and attach on the circuit pattern surface of the mask, thereby causing an image of the foreign objects to be transferred to the exposed substrate. Thus, defective products and decreased yields result. Hence, it is vital that the mask and the pellicle are free from the foreign objects.
A conventional pellicle pasting method for a conventional pellicle pasting apparatus will now be described with reference to FIG. 6.
To begin with, the pellicle pasting apparatus, after washing the mask during a mask washing step 1 of a mask washing unit, determines whether foreign objects larger than 0.5 .mu.m, such as dust and dirt (hereafter, the first foreign objects), which do not transmit light are attached on the mask during the first mask inspection step 2 of the first mask inspection unit, and if the first foreign objects are attached on the mask, the mask is washed again in the mask washing step 1 in the mask washing unit.
If the first foreign objects are not attached on the mask during the first inspection step 2 of the first inspection unit, the pellicle pasting apparatus pastes the pellicle on the mask during the pellicle pasting step 3 using the pellicle pasting unit. Here, the pellicle is inspected beforehand by the worker's eye to determine whether foreign objects are attached on the pellicle. Next, the pellicle pasting apparatus inspects whether the first foreign objects are attached on the pellicle pasted mask during the second inspection step 4 using the first inspection unit. If the first foreign objects are not attached on the pellicle pasted mask, the pellicle pasted mask is stored in a special mask case.
However, if the first foreign objects are attached on the pellicle pasted mask, the process steps from the mask washing step to the second mask inspection step 4 are repeated. Moreover, because the first inspection step of the first inspection unit and the second inspection step of the first inspection unit are actually the same inspection, the second inspection step is executed in the first inspection unit.
The pellicle pasting system comprises a mask washing unit, a first inspection unit and a pellicle pasting unit, wherein each processing unit is independent from the other. Hence, in the pellicle pasting system, the mask moves sequentially from the mask washing unit (mask washing step) to the first inspection unit (the first inspection step), to the pellicle pasting unit, to the first inspection unit (the second inspection step), and to the mask washing unit. After each step, a worker must store the mask or the pellicle pasted mask in the special mask case and carry the case to the subsequent process unit or to the previous process unit.
However, because the worker moves the mask or the pellicle pasted mask to the subsequent process step and to the previous process step, the probability of foreign objects attaching to the mask or the pellicle pasted mask increases even though the mask or the pellicle pasted mask is carried in the special mask case. Hence, it is difficult to maintain low contamination levels in the pellicle pasting system.
A proposed pellicle pasting system to resolve such problems moves the mask and the pellicle pasted mask from the first inspection unit to the pellicle pasting unit, and moves the mask and the pellicle pasted mask from the pellicle pasting unit to the first inspection unit automatically using a mask transport apparatus. In such pellicle pasting apparatus, all the mask movements except for movement between the mask washing unit and the first inspection unit are executed by the mask transport apparatus, thereby substantially reducing the possibility of foreign objects attaching to the mask and the pellicle pasted mask.
Also, the foreign objects which attach to the mask and the pellicle pasted mask and which are targeted as ones preventing improved yields are larger than 0.5 .mu.m which do not transmit light as described above. Hence, the first and the second inspection steps of the first inspection unit concerns with the attachment of such first foreign objects on the mask and the pellicle pasted mask. However, as mentioned above, integrated circuits are becoming increasingly finer in recent years. Thus, foreign objects, such as oil and the like which are semi-transparent, flat, partially light-transmitting foreign objects (hereafter, referred to as the second foreign objects), which previously were not the primary source problems when attached to the mask and the pellicle pasted mask, are beginning to be a large source of problems. Moreover, the traditional inspection by eye cannot detect the second foreign objects described above. Accordingly, a need has become apparent for automatic inspection using a specific inspection apparatus to determine whether foreign objects are attached to the pellicle.
Hence, a pellicle pasting apparatus has been proposed wherein a specific inspection apparatus is used to inspect foreign objects on the pellicle in addition to execution of the first inspection step to inspect to determine whether the first foreign objects are attached to the mask and the second inspection step to determine whether any second foreign objects are attached to the mask before and after the pasting of the pellicle on the mask. The process of this pellicle pasting method using a conventional pellicle pasting system will now be described with reference to FIG. 7.
To begin, the pellicle pasting system, after washing the mask during a mask washing step 10 of a mask washing unit, inspects whether any first foreign objects are attached to the mask in the first mask inspection step 11 using the first mask inspection unit. If first foreign objects are attached to the mask, the mask is washed again in the mask washing step 10. If first foreign objects are not attached to the mask, the apparatus moves to the second inspection step 12 using the second inspection unit.
During the second inspection step 12 inspection unit, the pellicle pasting system determines whether any second foreign objects are attached to the mask. If the second foreign objects are attached to the mask, the mask washing step 10 of the mask washing unit and the first inspection step 11 are repeated. The pellicle pasting system then pastes the pellicle on the mask during the pellicle pasting step 13 using a pellicle pasting unit if the second foreign objects are not attached to the mask.
As far as the foreign object inspection of the pellicle is concerned, the inspection is executed using a specific inspection apparatus during the pellicle inspection step 14 of the pellicle inspection unit. Then, the pellicle, which is determined not to have foreign objects, is moved to the pellicle pasting unit. Next, the pellicle pasting system determines whether any first foreign objects are attached to the pellicle pasted mask during the third inspection step 15 of the first inspection unit. Then, the pellicle pasting system determines whether any second foreign objects are attached to the pellicle pasted mask during the fourth inspection step 16 of the second inspection unit.
Here, the first inspection step 11 of the first inspection unit and the third inspection step 15 of the first inspection unit are the same type of inspections, and the second inspection step 12 of the second inspection unit and the fourth inspection step 16 of the second inspection unit are the same type of inspections. Hence, the third inspection step and the fourth inspection step are actually executed in the first inspection unit and the second inspection unit, respectively.
Moreover, each processing unit of the pellicle pasting system described above operates so that the mask stored in the original position of a mask storage case 17, such as the one shown in FIG. 8. For example in practice, a worker hand carries the case 17 after washing is completed by the mask washing unit to the first inspection unit, where the next process step takes place. In this case, two mask storage cases 17, one to supply the mask at each processing unit and the other to store processed mask are provided.
Moreover, during the first inspection step 11 of the first inspection unit, the mask for which the first foreign objects are determined to be attached is stored with the mask for which the first foreign objects are determined not to be attached in the original position of the mask storage case 17 and the worker is to separate the mask with foreign objects and carries them in separate storage case to the mask washing unit.
In the pellicle pasting system described above, mask movement from the first inspection unit to the second inspection unit and from the second inspection unit to the pellicle pasting unit is automatically done using a specific transport apparatus and not manually. The movement of the mask and the pellicle pasted mask other than between these processing units and from pellicle inspection unit to pellicle pasting unit is performed manually.
However, in the pellicle pasting system, the number of processing steps has increased compared to the pellicle pasting apparatus described earlier by the second inspection step and the fourth inspection step. Thus, the number of movements of the mask and the pellicle pasted mask increases by the same amount. Hence, the possibility that the foreign objects attach to the mask and the pellicle pasted mask increases with the increased the number of mask transfers and the pellicle pasted mask, thereby resulting in further difficulties in managing the degree of cleanliness for the pellicle pasting system. Accordingly, a which, in turn, causes substantial decline in yield of the pellicle pasted mask results.